This invention relates to a method and apparatus for generating heat from combustion of a fuel with a high sulphur and nitrogen content, having reduced emission of sulphur oxides, nitrogen oxides and unburnt combustible compounds. More particularly, it relates to the control and optimisation of desulphurisation in a entrained bed in a heat generator.
In protected zones strict regulations limit sulphur oxide emissions in gaseous effluent from heat generators and prohibit the use of fuels with a high sulphur content, which may moreover have definite economic advantages: this is the case with some coals of the lignite family and petroleum residues from refinery processes.
The technological background is illustrated by Patents U.S. Pat. Nos. 4,277,450, EP-A-0093063, EP-A-0211458 and DE-A-3235558. Patent EP-A-0362015 is also known, describing an apparatus for generating heat in a fluidised bed without any recycling of absorbent particles. Patent FR 2575272 on the other hand describes a method which includes recycling the particles the combusion chamber, where both combustion and desulphurisation of the fuel are carried out. None of these patents suggests the technical problem mentioned below.
It is known from French Patent 2636720 to reduce emissions of sulphur oxides and particularly sulphur dioxide from a heat generator which accepts fuels with a high sulphur content. A combination of three main elements is in fact used, forming a compact unit: a combustion chamber with cold walls, a heat recovery unit which absorbs the heat from the combustion gases and circulating particles, and an intermediate entrained bed having no appreciable internal heat exchange area, its function being to desulphurise the gases in transit between the upstream combustion chamber and the downstream heat exchanger. The technical problem generally encountered in this type of steam generator is thefact that the great majority of heaters are required to function at variable loads, so that steam production can be adapted to demand. Any change in the throughput of the heater, that is to say, any change in the flow rate of air and fuel, is inevitably accompanied by variation of the thermal profiles inside the actual heater, for the coefficients of heat exchange, by convection and a fortiori by radiation, are not directly proportional to the flow rate of fumes travelling through the heat exchange areas.
In the case of the heat generator described in Patent FR 2636720, a reduction in loads leads to a drop in the mean temperature of the desulphurisation chamber. The drop may be of 200 or even 300.degree. C., when the heat production of the generator is less than 30% of that under nominal operating conditions. The desulphurisation chamber has to operate within a temperature range which may be fairly narrow with some types of absorbent (e.g. 850.degree. to 950.degree. C. for some limestones), however, and hence desulphurisation performance may suffer greatly if the load is reduced. It is known that temperature variations may be considerably reduced if fumes collected after the last heat exchange zone are recycled to the combustion chamber. This solution could be envisaged to solve the problem of temperature maintainence in the desulphurisation chamber, but it involves a relatively high capital outlay which would be difficult to justify for an industrial scale heater with an output of only a few magawatt to some tens of megawatts.
To maintain the temperature of the desulphurisation chamber or to avoid too great a drop when the load is reduced, the proposed solution comprises recycling not the gases but at least part of the used absorbent which is recovered downstream of the final heat exchanger. A method has been discovered which solves the above problem and in particular enables high desulphurisation speeds to be obtained and the absorbent utilisation rate to be increased at a lower cost, whatever the operating speed of the heat generator.
Another object of the invention is to keep the temperature of the chamber for desulphurising the fumes resulting from combustion of the fuel at a level compatible with optimum operation of the generator, whatever its load.